This invention relates to a process for the removal of urea, ammonia, and carbon dioxide from dilute aqueous solutions.
In the preparation of urea from ammonia and carbon dioxide at elevated temperatures and pressures, a urea synthesis solution is formed which contains, in addition to product urea, a substantial quantity of free ammonia and non-converted ammonium carbamate. To recover the urea, the ammonium carbamate is first decomposed in one or more pressure steps, into ammonia and carbon dioxide which are then driven out of the solution, together with free ammonia, and usually recirculated. The final decomposition step generally results in an aqueous urea solution which still contains a quantity of dissolved ammonia and carbon dioxide, which are removed by expansion to atmospheric or even lower pressures. The remaining aqueous urea solution is then concentrated by evaporation and/or crystallization and subjected to further processing as required.
During the evaporation of the aqueous urea solution a gas mixture is formed which, in addition to water vapor, contains ammonia, carbon dioxide, and entrained fine droplets of urea. This gas mixture from the evaporation step is condensed, usually together with the gas mixture removed by the expansion of the aqueous urea solution, to atmospheric or lower pressure, and a portion of the process condensate thus obtained is returned to the process. For instance, this process condensate can be used for absorbing the gas mixture driven off in the final decomposition step. The remaining portion of the process condensate is discharged from the process.
This process condensate incorporates the various water streams fed into the process, including the steam used for operating the ejectors in the evaporation section, washing water, and flushing water applied to the stuffing boxes of the carbamate pumps. Additionally, for each mole of urea synthesized, one mole of water is formed. Thus, in a urea plant with a capacity of 1,500 tons of urea per day, 450 tons per day of water are formed. About 300 tons per day of additional water are fed into the process, so that roughly 750 tons of water in total must be discharged from the process.
This process condensate will generally contain approximately 2-9 percent by weight ammonia, 0.8-6 percent by weight carbon dioxide, and 0.3-1.5 percent by weight of urea. This represents important quantities of raw materials and product that ideally should be recovered. Moreover, if it is discharged as such, it would load the surface water into which it is discharged with waste to a degree no longer permitted by the governments of many countries. It is, therefore, necessary to remove a major portion of the ammonia and urea present prior to discharging this process condensate to the environment.
To accomplish this, the process condensate can be subjected to a treatment such as described in Industrial Wastes, September/October, 1976, at pages 44-47, wherein the process condensate, already freed of part of the ammonia and carbon dioxide by desorption at low pressure, is passed at a higher pressure into the bottom of a reaction column. In the reaction column, it is heated by means of steam, also fed into the bottom, resulting in the hydrolysis of the urea present. The solution thus obtained, having a reduced urea content, is removed from the top of the reaction column. In addition to a small quantity of nonhydrolyzed urea, this resulting solution also contains ammonia and carbon dioxide which are removed, after expansion of the solution to a lower pressure, in a second desorption column by stripping with steam. The gas mixture obtained in the second desorption column can be used as the stripping agent in the first desorption column. The bottom product liquid stream from the second desorption column is then discharged from the process after heat exchange with the process condensate to be treated.
Although this known process does succeed in removing the major portion of urea and ammonia from the process condensate, in practice this waste flow discharged into the environment will still contain about 50 ppm ammonia and 50 ppm urea. Furthermore, even with very long residence times, for which inefficiently large reaction columns would be required, it is impossible to reach a urea content lower than about 20-25 ppm.